1. Field of the Invention
The present invention relates generally to blinds and, more specifically, to a lift lock used in a blind to lock the lift rod.
2. Description of the Related Art
A conventional Venetian blinds uses a lift cord to control the extending status, and bladder tapes to share the weight of the blind slats. When receiving the blind, the lift cord starts to bear the weight of the blind slats. When the blind slats received in a stack between the headrail and the bottom rail, the lift cords bears the total weight of the blind slats. When lifting or lowering the blind slats, the user needs only to pull the suspending part of the lift cord outside the headrail. However, because the suspended part of the lift cord is exposed to the outside of the headrail and easily accessible by a child, the suspending part of the lift cord may be hung on a child""s head accidentally. In order to eliminate this problem, blinds with hidden lift cord are disclosed. A blind with hidden lift cord comprises a lift rod fastened pivotally with the inside of the headrail, and a spring mechanism mounted inside the headrail and coupled to the lift rod. The lift rod can be rotated to roll up or let off the lift cord, so as to further lift or lower the bottom rail of the blind. The spring power of the spring mechanism bears the weight of the bottom rail as well as the blind slats and is maintained in balance with the torque of the lift rod, enabling the blind to be positioned in the desired extending position. During operation, the user needs only to impart an upward or downward pressure to break the balance, i.e., when the user lowering or lifting the bottom rail and then releasing the hand from the bottom rail, the reversing force of the spring mechanism balances the torque of the lift rod, thereby keeping the blind in position. In order to keep the spring force of the spring mechanism in balance with the torque of the lift rod at different elevations, a variable adjusting means is provided in the spring mechanism to automatically regulate the reversing force of the spring mechanism subject to the elevation of the blind. Alternatively, the bottom rail may be made relatively heavier and the blind slats relatively lighter to control the variation of load within 15%. However, the aforesaid conventional designs cannot accurately lock the blind in position. When the spring power of the spring mechanism designed to be excessively high, the blind tends to be lifted slightly after pulled to the desired elevation, and cannot be set in the fully extended position. When the spring power of the spring mechanism designed to be insufficient or when the spring mechanism started to wear, the blind tends to be lowered slightly after pulled to the desired elevation, and cannot be fully received in the upper limit position.
Therefore, it is desirable to provide a lift control for blind that eliminates the aforesaid drawbacks.
It is one object of the present invention to provide a lift lock for blind, which is made in the form of an independent module.
It is another object of the present invention to provide a lift lock for blind, which achieves accurate positioning of the lift rod of the blind.
It is still another object of the present invention to provide a lift lock for blind, which is inexpensive to manufacture and easy to install.
It is still another object of the present invention to provide a lift lock for blind, which has a compact and simple structure that requires less installation space.
To achieve these objects of the present invention, the lift lock for locking position of a lift rod of a blind when a rotary driving force applied to the lift rod is dropped below a predetermined value comprises a shaft, a guide wheel, balls, a casing, a spring member and a retainer. The shaft is adapted to receive the lift rod of the blind. The balls are respectively received in respective sliding grooves around one side of the guide wheel which is sleeved onto the shaft. The retainer is coupled to the shaft. The spring member is sleeved onto the shaft for forcing an engagement portion of the shaft into engagement with a positioning opening of the casing. When the lift rod rotated by an external rotary driving force that surpasses the friction resistance between the shaft and the casing, the shaft is rotated with the lift rod. When the external rotary driving force dropped below the friction resistance between the shaft and the casing, the shaft is stopped to hold down the lift rod in position.